[libs] Skip AdvSimdEncode on Mono (#96829)

* [libs] Skip AdvSimdEncode on Mono

* Add guard around existing AdvSimdEncode callsite to exclude on Mono

* Add issue to guard

* Guard AdvSimdEncode logic for not Mono

* Make AdvSimdEncode inaccessible on Mono

* Update issue comment

Author: mdh1418

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs

@@ -85,6 +85,7 @@ public static unsafe OperationStatus EncodeToUtf8(ReadOnlySpan<byte> bytes, Span
                             goto DoneExit;
                     }
 
+#if !MONO // https://github.com/dotnet/runtime/issues/93081
                     end = srcMax - 48;
                     if (AdvSimd.Arm64.IsSupported && (end >= src))
                     {
@@ -93,6 +94,7 @@ public static unsafe OperationStatus EncodeToUtf8(ReadOnlySpan<byte> bytes, Span
                         if (src == srcEnd)
                             goto DoneExit;
                     }
+#endif
 
                     end = srcMax - 16;
                     if ((Ssse3.IsSupported || AdvSimd.Arm64.IsSupported) && BitConverter.IsLittleEndian && (end >= src))
@@ -489,6 +491,7 @@ private static unsafe void Avx2Encode(ref byte* srcBytes, ref byte* destBytes, b
             destBytes = dest;
         }
 
+#if !MONO // https://github.com/dotnet/runtime/issues/93081
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         [CompExactlyDependsOn(typeof(AdvSimd.Arm64))]
         private static unsafe void AdvSimdEncode(ref byte* srcBytes, ref byte* destBytes, byte* srcEnd, int sourceLength, int destLength, byte* srcStart, byte* destStart)
@@ -546,6 +549,7 @@ private static unsafe void AdvSimdEncode(ref byte* srcBytes, ref byte* destBytes
             srcBytes = src;
             destBytes = dest;
         }
+#endif // !MONO
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         [CompExactlyDependsOn(typeof(Ssse3))]